1. Field of the Invention
The present invention relates to a current balance circuit and a multiphase DC-DC converter and a current balance method thereof, and more particularly, to a current balance circuit and a multiphase DC-DC converter and a current balance method thereof capable of shifting a time of starting to increase a voltage of a ramp signal to perform the current balance, so as to maintain a slope and amplitude of the ramp signal to be constant to prevent a loop gain of the current balance technique from varying with output and input voltages.
2. Description of the Prior Art
With the electronic products developing rapidly, requirements for specification on power management chips of the electronic products become more rigorous, especially on those for the microprocessors with high performance. In addition to high efficiency, the power management chips are also required to have an extreme high current driving capability and low ripples on the output voltage. In comparison with the conventional single channel DC-DC converter, a multiphase DC-DC converter has multiple parallel channels, and can distribute currents equally to each channel to provide a larger current. Then the multiphase DC-DC converter subtracts the output voltage by a reference voltage to be amplified via an amplifier to generate a voltage error signal, which will be compared with ramp signals with multiphase to adjust the duty cycles of pulse width modulation signals to control the power transistors of each channel, so as to adjust the currents of each channel. As a result, the multi-channel can provide power with multiphase to a load simultaneously, and thus can reduce ripples on the output voltage by well controlling the power with multiphase. Therefore, the multiphase DC-DC converter is most suitable for the high quality power management chips.
However, since there may be mismatches between the power transistor, inductor, and controller in each channel and on the printed circuit board (PCB), such that the currents of each channel may be imbalance and unequal. In addition to efficiency reduction, when the imbalance is severe, most of the currents flow through the same channel, and hence heats cannot be dissipated and the channel will be burnt. Therefore, the conventional multiphase DC-DC converter still needs a current balance technique to make the currents equally distributed to each channel to avoid the above situation to occur.
In detail, the conventional current balance technique applied in the multiphase DC-DC converters detects magnitudes of the currents flowing through each channel and calculates the average current, and then subtracts the current flowing through each channel by the average current to obtain a difference between the current flowing through each channel and the average current, so as to generate a current error signal. In the conventional current balance technique, the above current error signal is added to a voltage error signal, and thus each channel is corresponding to a voltage error signal, which can be utilized for controlling the output voltage and correcting the imbalance of the current. However, since this method combines the output voltage regulating and the current balance control on a voltage error signal, the control is more complex.
On the other hand, regarding the conventional current balance technique of adjusting slopes (i.e. amplitudes) of ramp signals, please refer to FIG. 1A to FIG. 1C. FIG. 1A is a waveform diagram of not performing the current balance control, and FIG. 1B and FIG. 1C are waveform diagrams of increasing and reducing amplitudes of ramp signals to perform the current balance, respectively. As shown in FIG. 1A, in a channel of the multiphase DC-DC converter, when current of the channel and the average current of all channels are equal and the current balance control is not performed, a voltage error signal VC is directly compared with a ramp signal VRamp to obtain a duty cycle D required in this channel for stabilizing the output voltage. On the other hand, as shown in FIG. 1B, when the current of the channel is greater than the average current, the current of the channel should be reduced to achieve the current balance, and thus the slope (i.e. amplitude) of the ramp signal VRamp is increased according to the current error signal first, and then the ramp signal VRamp is compared with the voltage error signal VC to reduce the duty cycle D, so as to reduce the current of the channel. On the contrary, as shown in FIG. 1C, when the current of the channel is less than the average current, the current of the channel should be increased to achieve the current balance, and thus the slope (i.e. amplitude) of the ramp signal VRamp is reduced according to the current error signal first, and then the ramp signal VRamp is compared with the voltage error signal VC to increase the duty cycle D, so as to increase the current of the channel. In such a situation, for the channels with different currents, the current balance technique can simply adjust the slopes (i.e. amplitudes) of the ramp signals according to the currents of the channels to achieve the current balance.
However, regarding the above current balance technique of adjusting the slopes (i.e. amplitudes) of the ramp signals, loop gain of the current balance may change with variation of the duty cycle D. In such a condition, when the input voltage is far greater than the output voltage, the loop gain of the current balance is low, and performance of the current balance is worse, e.g. when the voltage error signal VC is lower, the duty cycle D has less variation with the same degree of variation in the slope of the ramp signal VRamp; when the input voltage is close to the output voltage, the loop gain of the current balance is high, and performance of the current balance is better, e.g. when the voltage error signal VC is higher, the duty cycle D has greater variation with the same degree of variation in the slope of the ramp signal VRamp, but the system stability is worse. Thus, there is a need for improvement of the prior art.